Use this pattern to combine two ideas. For the example at top, the two ideas are:

I had a chocolate cake for dessert. The chocolate cake was good but nothing special.

Here are a few more examples of how you can use "which":

We're planning a trip to Australia, which we've never been to. Eric's mother is coming to visit us, which is great. It was less than $100 for the four of us, which is quite reasonable.